Google Plus will be open for everyone on July 31st 2011

Google Plus currently can be joined only via invitation. Actually its under Beta phase, So Google is testing its functioning before opening it for all.
Good news is that Google plus is going to open for all on 31/july/2011

According to Google Plus team, it can be a day or two sooner but not late.

So, from 31/july/2011 you do not need any invitation to join Google Plus.